1. Field of the Invention
This invention relates to asynchronous transfer mode networks and, more particularly, to a system and method for avoiding cell loss due to quality of service contract bandwidth usage constraints.
2. Description of the Related Art
The development of broadband integrated services digital networks (BISDN) based on asynchronous transfer mode (ATM) switching technology has the potential for ultimately replacing the plurality of existing networks presently required to carry voice, data, and video traffic. ATM is a high-speed packet switching technique employing short, fixed-length cells that are statistically multiplexed over virtual connections. Using ATM, different traffic types received in various user formats are segmented into fixed length cells that are transported to and reassembled into the original format at the destination node.
To achieve universality, standards and recommendations for service for ATM-based networks are promulgated by the International Telecommunications Union (ITU). In particular, the ITU-I Recommendation I.371 specifies a method for policing traffic through an ATM-based network via the use of traffic management contracts. A traffic contract is an agreement between an ATM user and the network provider regarding the quality of service (QoS) the ATM user can expect under specified conditions. The contracts are designed to ensure the integrity of information transport as well as an acceptable level of delay for voice and video traffic. One parameter specified in the traffic contract is the amount of guaranteed bandwidth. When a user exceeds the agreed-upon bandwidth, the excess cells are liable to be discarded if the network is congested. In order to ensure the integrity of data and information transport, users are therefore typically required to pay for a larger bandwidth allocation, which can be relatively expensive.
Accordingly, it is desirable to provide a system and method for ensuring cell integrity at optimal costs for QoS traffic contracts in ATM networks.
These problems are solved in large part by a system and method according to the present invention in which excess cells, or cells whose transfer is expected to cause the user to exceed the QoS traffic contract bandwidth limit (or any predetermined QoS or bandwidth threshold), are transferred to a destination node via an alternate or secondary network. More particularly, user traffic received at a first ATM node (for example, an end user node) is or has been classified according to traffic type, e.g., voice, data, or video (with their corresponding characteristics). Prior to packetizing the data streams into the ATM fixed length packets, the ATM node determines whether the QoS traffic contract bandwidth limit has been exceeded or is likely to be exceeded. If so, then the corresponding traffic is transmitted to the destination node via an appropriate alternate network. For example, voice traffic that would cause the QoS threshold to be exceeded is transferred to a PBX and, ultimately, to the public switched telephone network (PSTN). Once the bandwidth utilization of the ATM network falls below the threshold level, all traffic is routed back through the ATM network.
A method according to one embodiment of the present invention comprises defining a quality of service threshold for an ATM network. The quality of service threshold is less than or equal to, preferably somewhat lower, than a bandwidth limit defined in a quality of service traffic contract. The method further comprises receiving user data at a first ATM node and identifying the user data according to any of a plurality of data types. The method further comprises receiving information from a network manager identifying whether a usage of the ATM network has exceeded the QoS threshold. At least a portion of the user data are routed via a corresponding secondary network if the transmission of the user data would cause the QoS threshold to be exceeded, or if it has already been exceeded. The secondary network is chosen to correspond to the type of user data.
A method for rerouting data in an asynchronous transfer mode network according to another embodiment of the present invention comprises defining a quality of service threshold for the ATM network and receiving user data at a first ATM node. Again, the data are identified according to any of a plurality of user data types. Information is received from a network manager identifying whether a usage of the ATM network has exceeded the QoS threshold. Based on past usage patterns, and the received information, the method farther comprises predicting when network usage is expected to exceed the QoS threshold. At least a portion of the user data are routed to a corresponding secondary network during periods when the usage is predicted to exceed the QoS threshold. Information used to predict when the usage exceeds the QoS threshold includes the source of the user data which is being transmitted and the type of user data which is being transmitted. The data transmitted via the ATM network and the secondary network are reintegrated at a secondary destination ATM node.
An ATM node according to one embodiment of the present invention comprises a user network interface for receiving user traffic, such as voice, video or data. The user network interface is coupled to a control CPU, which in turn is coupled to receive one or more control signals from an external network management system, preferably an ATM network manager. The user network interface is coupled to a switch to provide switching of the received traffic to a plurality of network interface modules. One of the network interface modules preferably comprises an ATM network interface module. The remaining network interface modules connect the ATM node with one or more secondary networks, such as an integrated services digital network (ISDN), the public switched telephone network (PSTN), a wireless network, or a frame relay network. When traffic is received at the user network interface, a control signal from the control CPU (responsive to the network management system) indicates whether the QoS threshold has been or is likely to be exceeded. If not, the switch switches the traffic to the ATM network interface module and the traffic is transferred to a destination node via the ATM network. If, however, the QoS threshold has been or is likely to be exceeded, the switch switches at least a portion of the incoming traffic to one or more of the secondary network interface modules. Similarly, data may be received at the ATM node via one of the secondary networks and switched to the user network interface and, ultimately, provided to the user.